Phosphocreatine is the reservoir of high energy phosphate in tissues and is also implicated in mitochondrial electron transport. The first reaction in the biosynthesis of creatine is the formation of guanidinoacetic acid, which is catalyzed by the enzyme L-arginine:glycine amidinotransferase (transamidinase). The transamidinase activities of rat kidney are many fold greater than found in the other tissues (except pancreas) and are under dietary and hormonal control. The proposed research is the investigation of the requirement of both growth hormone and thyroxine for the maintenance of rat kidney transamidinase protein and of the repression of transamidinase activities by creatine or guanidinoacetic acid in the diet. This information should contribute to the knowledge on the action of growth hormone and thyroxine at the molecular level and on hormonal and nutritional interrelationships in creatine metabolism. We have developed a procedure for the isolation of rat kidney transamidinase in a homogeneous state. Monospecific antibodies to the enzyme have been prepared. An excellent correlation has been found between the amounts and the activities of transamidinase in kidneys from hypophysectomized rats and hypophysectomized rats injected with growth hormone and thyroxine. The amounts of transamidinase protein will be determined in kidneys from rats fed diets known to markedly alter the enzyme activities. The rates of synthesis and degradation of the enzyme will be determined whenever the activities correlate with enzyme protein levels. Attempts will be made to determine the relative amounts of mRNA for transamidinase whenever alteration in the rates of enzyme synthesis are indicated. All the information on alterations of transamidinase activities in various dietary or hormonal states has been obtained with whole animals or rats with excised endocrine glands. Attempts will be made to determine if the hormonal or nutritional factors have an effect on transamidinase in cultured normal rat kidney cells.